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39 votes

Why did they decide to hit Dimorphos in the retrograde direction rather than prograde; was it a "coin-toss" or were there implications for observing?

The original orbital period of Dimorphos was approximately 11.92 hours. The DART scientists weren't sure how much momentum was going to be exchanged due to the collision since they weren't sure how ...
Connor Garcia's user avatar
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33 votes

Why did Theia create only one moon if it struck Earth?

Most simulations generate multiple moonlets and streams that then fall into Earth, merge into the moon or escape (e.g. like this or this). The initial orbit tends to be close to the Earth. This means ...
Anders Sandberg's user avatar
32 votes
Accepted

What would happen if a small black hole fell into a star?

How much matter can a small black hole absorb? This determines the answer to your questions. A simple approximation is that moving black holes absorb everything they pass through out to a radius $kR_S$...
Anders Sandberg's user avatar
29 votes
Accepted

Is it possible for the Milky Way and Andromeda to get ejected upon colliding instead of merging?

A full merger may not occur on that timescale. The encounter in 4.5 billion years will take place but the galactic centers may be far enough apart that it is a "glancing blow". Regardless, ...
ProfRob's user avatar
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21 votes

What are the odds that the Sun hits another star?

TL;DR: Virtually zero. The distances between stars are HUGE and stars are tiny compared to the astronomical scales of distance between neighboring stars. The sun, is about 0.0000001 or one ten-...
WarpPrime's user avatar
  • 6,673
18 votes

What would happen if Venus and Earth collided?

This is the kind of thing that computer simulations are done for, but I am not aware of any that were specifically for a Venus-sized body hitting the earth. The specific details would depend very much ...
Mark Foskey's user avatar
  • 3,916
18 votes

What would happen if Venus and Earth collided?

The outcome of a Venus - Earth collision depends on the impact speed. At a relatively low speed, the matter is likely to condense to form a new planet. But if the speed is high enough the planets will ...
PM 2Ring's user avatar
  • 14.9k
16 votes

Does the barycenter of an n-body system remain constant during collisions?

Conservation of momentum dictates that if there are no forces external to the system then the barycentre will not be accelerated. That means the second derivative of its position with respect to time ...
ProfRob's user avatar
  • 153k
12 votes

What are the odds that the Sun hits another star?

As fasterthanlight says, the probability of our Sun colliding with another star in the galaxy is virtually nil. In fact, the probability of any star in the galaxy colliding with another (unrelated) ...
PM 2Ring's user avatar
  • 14.9k
11 votes

What would happen if a small black hole fell into a star?

@AndersSandberg's answer surprised me so I wanted to poke around some more, to see what could a determined attacker achieve. So here's what would happen if we put a BH in orbit inside a star so that ...
Filip Sondej's user avatar
10 votes
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Head on collision of two black holes

Well the spin is easy. Your system has zero angular momentum, so the spin will be zero. I think there will also be no gravitational radiation. for the slightly technical reason that gravitational ...
Steve Linton's user avatar
  • 10.3k
9 votes

How do rocky planet break up? Would they fragment into "a gazillion" rocky pieces pieces, or crack open like an egg?

Energy Intro I think any good discussion of planetary destruction should start by talking about energy! Our SI base unit of energy is a Joule, which is defined as $\mathrm{J} = \mathrm{kg} \mathrm{m}^...
Connor Garcia's user avatar
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8 votes
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Why didn't Kessler Syndrome prevent planet formation?

I'll get to your question shortly, but first let's talk about the asteroids. Asteroid sizes roughly follow a power law, $n \propto D^{-2.5}$, where $n$ is the number of asteroids larger than diameter $...
PM 2Ring's user avatar
  • 14.9k
8 votes
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Had Earth lost the Moon early, would we be able to determine it once had a companion?

The collision between the Proto-Earth and "Theia", the hypothetical planet responsible for the Moon's formation, is thought to have given us the essential elements for life to form, since it ...
4NT4R3S's user avatar
  • 1,015
7 votes
Accepted

If two stars collide, what is the probability that they merge to form a single star?

Fairly good. Two stars of mass $M$ falling from infinity straight towards each other until they merge at distance $2R$ will get kinetic energy $GM^2/R$. This is a lot, for two suns it is $1.8978\times ...
Anders Sandberg's user avatar
7 votes

Comet Bernardinelli-Bernstein; could it be a "planet-killer" and what could prevent it from getting close to us?

Objects in space follow the laws of gravity. Since the sun is by far the biggest object in the solar system, nearly all objects (except moons) are in elliptical orbits around the sun. There are ...
James K's user avatar
  • 123k
7 votes

Why did they decide to hit Dimorphos in the retrograde direction rather than prograde; was it a "coin-toss" or were there implications for observing?

Here's a thought that is too long to be shared in a comment. In this question and answer, DART crash on Dimorphos: computation of orbital period change I tried to compute the change of the orbital ...
Sextus Empiricus's user avatar
7 votes
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Does the Final-Parsec problem apply to stellar collisions?

The "final parsec problem" describes the difficulty in getting two supermassive black holes close enough together that their merger timescale due to gravitational wave emission becomes ...
ProfRob's user avatar
  • 153k
6 votes
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Black hole collision animation: What are these extrusions?

The shape of the surface shown in the video is a depiction of the spacial curvature of the spacetime. (The relationship with time are depicted separately by the arrows and the colors.) More ...
TimRias's user avatar
  • 2,720
6 votes

Latest Planet Collision 2019

If you are talking specifically about our solar system, then no. There were many collisions during planetary formation, and there is the fairly well regarded theory that our moon was formed by a ...
Rory Alsop's user avatar
  • 5,102
6 votes

What is Earth believed to have looked like before the collision with Theia?

Greenwood et al suggest that Earth had a lot of its water (maybe upto 70%) before Theia, but I can't find anything definite saying that it was liquid, although several sites reporting on the Greenwood ...
PM 2Ring's user avatar
  • 14.9k
6 votes
Accepted

If two fast asteroids collided, would the temperature and pressure be high enough to cause nuclear fusion?

A different approach, ignoring any details of the collision. Note extreme simplifications will occur, do not try this at home... The easiest hydrogen isotope fusion process is D-T, with a cross ...
Jon Custer's user avatar
5 votes

What are the odds that the Sun hits another star?

Imagine that that there are 2 pigeons on LSD flying around the world. What are the chances that they would collide? Colliding stars are less likely because if the stars were as big as pigeons, their ...
bandybabboon's user avatar
  • 4,250
5 votes

Why collisions between earth orbiting satelites do occur?

"All earth orbiting satellites should have the same velocity" is not true. Kepler's Laws merely state that an object in a circular orbit at a particular altitude must have a particular speed....
Nuclear Hoagie's user avatar
5 votes
Accepted

What would happen to the Sun's evolution if it collided with another yellow dwarf?

If the Sun collided with another star about the same mass, then its mass would be slightly less than 2 solar masses, as some material would be ejected away from the merger. This would result in an A-...
WarpPrime's user avatar
  • 6,673
5 votes

When a rogue brown dwarf star hits a white dwarf star, will the collision at most cause a nova?

Collisions between a rogue brown dwarf and any other star would be very rare because the space between them is so vast. I don't want to say it'd never happen, but it would be a rare event. It's ...
userLTK's user avatar
  • 24.1k
5 votes
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How frequently are other planets in the Solar system hit by meteorites?

Moon Even though the Moon obviously isn't a planet, it's a good place to start as most of the observations of meteorites hitting other bodies has been focussed on the Moon. NASA runs the Meteoroid ...
Barry Jenakuns's user avatar
5 votes

What would happen if a small black hole fell into a star?

The existing answers haven't really addressed your point 2, so I'll cover that here. The possibly-surprising thing is that it's basically impossible to shoot a black hole at a star such that it won't ...
N. Virgo's user avatar
  • 150
4 votes

Small changes to Earth's orbit caused by a large impact on the Moon

... and still remain within the habitable zone? Everything perturbs all the time. Every change in the distribution of mass in the solar system (or in the universe) perturbs the orbit of the Earth... ...
James K's user avatar
  • 123k

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